A typical cellular wireless network includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which user equipment devices (UEs) can operate and engage in air-interface communication with the cellular wireless network. Each base station may then be coupled with network infrastructure that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. Within this arrangement, a UE operating in a coverage area of the cellular wireless network can engage in communication, via the cellular wireless network, with remote network entities or with other UEs operating in the cellular wireless network.
The cellular wireless network may operate in accordance with a particular air-interface protocol or “radio access technology,” examples of which include Long Term Evolution (using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), and Global System for Mobile Communications (GSM), among others. Generally, the agreed air-interface protocol may define a downlink (or forward link) for carrying communications from the base stations to UEs and an uplink (or reverse link) for carrying communications from UEs to the base stations.
In general, the air interface between a base station and served UEs will define a limited number of resources for carrying data communications.
By way of example, an LTE air interface operates on a carrier frequency that has a finite frequency bandwidth, such as 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz, for instance. The LTE air interface is then divided over time into a continuum of 10-millisecond frames each defining ten 1-millisecond subframes or TTIs, and each TTI is divided over time into 14 symbol segments in which data can be transmitted. Further, the carrier frequency bandwidth is divided into a sequence of 15 kHz subcarriers, with each subcarrier in a given symbol time segment defining a resource element in which data can be communicated using an applicable modulation scheme that can accommodate communication of up to a certain number of bits per resource element.
With this arrangement, the LTE air interface thus defines an array of resource elements for carrying data. Further, each TTI is divided into two 0.5-millisecond timeslots, and each timeslot is divided into physical resource blocks (PRBs) each being 12-subcarriers (i.e., 180 kHz) wide and 0.5-milliseconds in duration, thus encompassing 84 resource elements. Depending on the carrier bandwidth, the LTE air interface thus defines a limited number of PRBs per TTI. And depending on the modulation scheme, the LTE air interface can accommodate a limited number of bits communicated per resource element and thus per PRB.
In a representative cellular network, a base station may then employ a scheduler to dynamically allocate resources for communication of bearer data in PRBs between the base station and served UEs. In particular, for each UE, the base station could evaluate the UE's channel conditions (e.g., based on channel condition reports from the UE) and, based at least on the channel conditions, could decide a modulation and coding scheme (MCS) to serve the UE with, where the MCS could define (i) a modulation scheme establishing how many bits will be represented per resource element and (ii) a coding rate indicating how many of the bits represent actual data as opposed to error-correction coding bits. And for each UE, on a per TTI basis, the base station could allocate particular PRBs to carry data to or from the UE using the determined MCS. For a given TTI, the base station could then engage in signaling with the served UEs to designate the allocated PRBs and determined MCSs, and communication could proceed accordingly.
For instance, as the base station receives data for transmission over the air to various UEs, the base station may operate on a per-TTI basis to allocate particular PRB(s) for carrying data to particular UEs using the MCSs determined for those UEs based on their channel conditions. For each UE, the base station could then transmit to the UE a control message that designates the PRBs and the determined MCS, and the base station may accordingly transmit the data to UE in the designated PRBs and using the designated MCS. Thus, each UE could then receive the modulated transmission in the indicated PRBs and could demodulate the transmission in accordance with the indicated MCS, thus receiving the underlying data (subject to any need for retransmission).
And likewise, as UEs have data to transmit over the air to the base station, the UEs may send scheduling requests to the base station, and the base station may similarly operate on a per-TTI basis to allocate particular PRB(s) for carrying data from particular UEs using the MCSs determined for those UEs. For each UE, the base station may then transmit to the UE a control message that designated the PRBs and the determined MCS. And the UE may then accordingly transmit to the base station in the designated PRBs and using the designated MCS. Thus, the base station could receive the modulated transmission in the indicated PRBs from each UE and could demodulate the received transmission in accordance with the indicated MCS, thus receiving the underlying data (also subject to any need for retransmission).
In addition, the LTE air interface also reserves certain resource elements per TTI for special use rather than for carrying bearer data between the base station and served UEs. For instance, on the downlink, the first one, two, or three symbol time segments per TTI are reserved for control channel use (e.g., for transmitting PRB scheduling directives), various resource elements distributed throughout each TTI are reserved to provide a broadcast reference signal that UEs can evaluate to detect and determine coverage strength and quality, and other resource elements are reserved for other uses. And on the uplink, an upper portion and a lower portion of the frequency bandwidth per TTI are reserved for control channel use. This reservation of resource elements for other uses thus further limits resources on the air interface for use to carry data to and from UEs.